Description of naturally fractured reservoirs, combined with the knowledge of the physics of multi-phase flow provide the basis for understanding and forecasting the performance of these reservoirs. Recent advances in understanding of major forces (capillary and gravity, particularly when gravity drainage is the dominant mechanism) have contributed significantly to describing the flow in fractured porous media. However, the knowledge of main pertinent fracture parameters, that is, fracture capillary pressure (Pcf) and fracture relative permeability (krf), which affect flow in fractures and its interaction with blocks, is still associated with major uncertainties.
Naturally fractured reservoirs contain a significant portion of the world’s oil reserves which also represent a significant geological storage potential for CO2 as a means of greenhouse gas reduction. Geological CO2 storage in oil reservoirs, are most likely target for CO2 mitigation practices because of a number of reasons including additional economic benefit through EOR, existence of abundant characterisation data and utilising at least part of the existing infrastructure.
Despite a significant amount of research carried out on CO2 injection, the understandings of the interactions between CO2 and oil in fractured reservoir is at its early stages. Effects of diffusion and dispersion of CO2-gas from fractures into the matrix and the subsequent flow of oil from the matrix into surrounding fractures are important outstanding issues.
Gravity Drainage in Naturally Fractured Reservoirs
A combined experimental and theoretical study was initiated to gain further understanding of the capillary continuity across a stack of matrix blocks and its effect on the oil recovery. A series of gravity drainage experiments using a centrifuge were performed. In these experiments the effect of different possible flow mechanisms, i.e., film flow, liquid bridges formed independent of contact points and those formed supported by contact points were studied in a systematic manner. The recovery data for these experiments are shown in Figure 1. The results clearly highlight the existence of a strong capillary continuity which is mainly due to contribution of liquid bridges formed at contact point.
Figure 1: Recovery data from the six conducted experiments. In experiment one, recovery from 4 cm core plug, 1a refers to a speed of 1800 rpm, which is then increased to 2700 rpm and 1b refers to the speed of 2700 rpm. In Experiments 2 and 6 two pieces obtained by cutting the core in half were in direct contact. Experiments 3 to 5, a number of 0.1 mm diameter ring were used to separate the two pieces.
The effect of physical properties of fracture particularly the shape of Pcf and krf curves were determined by numerically simulating the recovery data of some of the performed experiments.
In this research program it is proposed to extend this study to determine Pcf and krf curves (controlled by the liquid bridges formed at contacting points) as a function of contact area between the blocks. The effect of presence of supercritical CO2 in fractures with its high solvency power to extract hydrocarbon components from the matrix is also studied. This multi-physics (flow and mass transfer) process should be investigated from the viewpoint of increasing both oil recovery and CO2 storage capacity of these reservoirs. This work will be carried out through mathematical modeling and also experimental verification.
Publications
2005
- Jamiolahmady M., Danesh A. and Sorosh H.: 2005, Gravity drainage flow in fractured porous medium, IASME Transactions, Issue 1, Volume 2, pp 124-130, also in Proceedings of WSEAS/IASME International Conference on fluid mechanics, 20-22 January, Udine, Italy.
2007
- Ngoc, S.L., Jamiolahmady, M., Questiaux, J.M. and Sohrabi, M.: Jean-Marie An Integrated Geology and Reservoir Engineering Approach for Modelling and History Matching of a Vietnamese Fractured Granite Basement Reservoir, To be presented in the SPE Europec Conference, SPE 107141, London, June 2007.